1. Field
The present disclosure is directed to compounds that are substantially active in the gastrointestinal tract to inhibit NHE-mediated antiport of sodium ions and hydrogen ions, and the use of such compounds in the treatment of disorders associated with fluid retention or salt overload and in the treatment of gastrointestinal tract disorders, including the treatment or reduction of pain associated with a gastrointestinal tract disorder.
2. Description of the Related Art
Disorders Associated with Fluid Retention and Salt Overload
According to the American Heart Association, more than 5 million Americans have suffered from heart failure, and an estimated 550,000 cases of congestive heart failure (CHF) occur each year (Schocken, D. D. et al., Prevention of heart failure: a scientific statement from the American Heart Association Councils on Epidemiology and Prevention, Clinical Cardiology, Cardiovascular Nursing, and High Blood Pressure Research; Quality of Care and Outcomes Research Interdisciplinary Working Group; and Functional Genomics and Translational Biology Interdisciplinary Working Group Circulation, v. 117, no. 19, p. 2544-2565 (2008)). The clinical syndrome of congestive heart failure occurs when cardiac dysfunction prevents adequate perfusion of peripheral tissues. The most common form of heart failure leading to CHF is systolic heart failure, caused by contractile failure of the myocardium. A main cause of CHF is due to ischemic coronary artery disease, with or without infarction. Long standing hypertension, particularly when it is poorly controlled, may lead to CHF.
In patients with CHF, neurohumoral compensatory mechanisms (i.e., the sympathetic nervous system and the renin-angiotensin system) are activated in an effort to maintain normal circulation. The renin-angiotensin system is activated in response to decreased cardiac output, causing increased levels of plasma renin, angiotensin II, and aldosterone. As blood volume increases in the heart, cardiac output increases proportionally, to a point where the heart is unable to dilate further. In the failing heart, contractility is reduced, so the heart operates at higher volumes and higher filling pressures to maintain output. Filling pressures may eventually increase to a level that causes transudation of fluid into the lungs and congestive symptoms (e.g., edema, shortness of breath). All of these symptoms are related to fluid volume and salt retention, and this chronic fluid and salt overload further contribute to disease progression.
Compliance with the medication regimen and with dietary sodium restrictions is a critical component of self-management for patients with heart failure and may lengthen life, reduce hospitalizations and improve quality of life. Physicians often recommend keeping salt intake below 2.3 g per day and no more than 2 g per day for people with heart failure. Most people eat considerably more than this, so it is likely that a person with congestive heart failure will need to find ways to reduce dietary salt.
A number of drug therapies currently exist for patients suffering from CHF. For example, diuretics may be used or administered to relieve congestion by decreasing volume and, consequently, filling pressures to below those that cause pulmonary edema. By counteracting the volume increase, diuretics reduce cardiac output; however, fatigue and dizziness may replace CHF symptoms. Among the classes or types of diuretics currently being used is thiazides. Thiazides inhibit NaCl transport in the kidney, thereby preventing reabsorption of Na in the cortical diluting segment at the ending portion of the loop of Henle and the proximal portion of the distal convoluted tubule. However, these drugs are not effective when the glomerular filtration rate (GFR) is less than 30 ml/min. Additionally, thiazides, as well as other diuretics, may cause hypokalemia. Also among the classes or types of diuretics currently being used is loop diuretics (e.g., furosemide). These are the most potent diuretics and are particularly effective in treating pulmonary edema. Loop diuretics inhibit the NaKCl transport system, thus preventing reabsorption of Na in the loop of Henle.
Patients that have persistent edema despite receiving high doses of diuretics may be or become diuretic-resistant. Diuretic resistance may be caused by poor availability of the drug. In patients with renal failure, which has a high occurrence in the CHF population, endogenous acids compete with loop diuretics such as furosemide for the organic acid secretory pathway in the tubular lumen of the nephron. Higher doses, or continuous infusion, are therefore needed to achieve entrance of an adequate amount of drug into the nephron. However, recent meta-analysis have raised awareness about the long-term risk of chronic use of diuretics in the treatment of CHF. For instance, in a recent study (Ahmed et al., Int J Cardiol. 2008 Apr. 10; 125(2): 246-253) it was shown that chronic diuretic use was associated with significantly increased mortality and hospitalization in ambulatory older adults with heart failure receiving angiotensin converting enzyme inhibitor and diuretics.
Angiotensin-converting enzyme (“ACE”) inhibitors are an example of another drug therapy that may be used to treat congestive heart failure. ACE inhibitors cause vasodilatation by blocking the renin-angiotensin-aldosterone system. Abnormally low cardiac output may cause the renal system to respond by releasing renin, which then converts angiotensinogen into angiotensin I. ACE converts angiotensin I into angiotensin II. Angiotensin II stimulates the thirst centers in the hypothalamus and causes vasoconstriction, thus increasing blood pressure and venous return. Angiotensin II also causes aldosterone to be released, causing reabsorption of Na and concomitant passive reabsorption of fluid, which in turn causes the blood volume to increase. ACE inhibitors block this compensatory system and improve cardiac performance by decreasing systemic and pulmonary vascular resistance. ACE inhibitors have shown survival benefit and conventionally have been a treatment of choice for CHF. However, since ACE inhibitors lower aldosterone, the K-secreting hormone, one of the side-effects of their use is hyperkalemia. In addition, ACE inhibitors have been show to lead to acute renal failure in certain categories of CHF patients. (See, e.g., C. S. Cruz et al., “Incidence and Predictors of Development of Acute Renal Failure Related to the Treatment of Congestive Heart Failure with ACE Inhibitors,” Nephron Clin. Pract., v. 105, no. 2, pp c77-c83 (2007)).
Patients with end stage renal disease (“ESRD”), i.e., stage 5 chronic kidney failure, must undergo hemodialysis three times per week. The quasi-absence of renal function and ability to eliminate salt and fluid results in large fluctuations in body weight as fluid and salt build up in the body (sodium/volume overload). The fluid overload is characterized as interdialytic weight gain. High fluid overload is also worsened by heart dysfunction, specifically CHF. Dialysis is used to remove uremic toxins and also adjust salt and fluid homeostasis. However, symptomatic intradialytic hypotension (SIH) may occur when patients are over-dialyzed. SIH is exhibited in about 15% to 25% of the ESRD population (Davenport, A., C. Cox, and R. Thuraisingham, Blood pressure control and symptomatic intradialytic hypotension in diabetic haemodialysis patients: a cross-sectional survey; Nephron Clin. Pract., v. 109, no. 2, p. c65-c71 (2008)). Like in hypertensive and CHF patients, dietary restrictions of salt and fluid are highly recommended but poorly followed because of the poor palatability of low-salt food.
The cause of primary or “essential” hypertension is elusive. However, several observations point to the kidney as a primary factor. The strongest data for excess salt intake and elevated blood pressure come from INTERSALT, a cross-sectional study of greater than 10,000 participants. For individuals, a significant, positive, independent linear relation between 24-hour sodium excretion and systolic blood pressure was found. Higher individual 24-hour urinary sodium excretions were found to be associated with higher systolic/diastolic blood pressure on average, by 6-3/3-0 mm Hg. Primary hypertension is a typical example of a complex, multifactorial, and polygenic trait. All these monogenic hypertensive syndromes are virtually confined to mutated genes involving gain of function of various components of the renin-angiotensin-aldosterone system, resulting in excessive renal sodium retention. In a broad sense, these syndromes are characterized by increased renal sodium reabsorption arising through either primary defects in sodium transport systems or stimulation of mineralocorticoid receptor activity (Altun, B., and M. Arici, 2006, Salt and blood pressure: time to challenge; Cardiology, v. 105, no. 1, p. 9-16 (2006)). A much larger number of controlled studies have been performed on hypertensive subjects during the last three decades to determine whether sodium reduction will reduce established high blood pressure. Meta-analyses of these studies have clearly shown a large decrease in blood pressure in hypertensive patients.
In end stage liver disease (ESLD), accumulation of fluid as ascites, edema or pleural effusion due to cirrhosis is common and results from a derangement in the extracellular fluid volume regulatory mechanisms. Fluid retention is the most frequent complication of ESLD and occurs in about 50% of patients within 10 years of the diagnosis of cirrhosis. This complication significantly impairs the quality of life of cirrhotic patients and is also associated with poor prognosis. The one-year and five-year survival rate is 85% and 56%, respectively (Kashani et al., Fluid retention in cirrhosis: pathophysiology and management; QJM, v. 101, no. 2, p. 71-85 (2008)). The most acceptable theories postulate that the initial event in ascites formation in the cirrhotic patient is sinusoidal hypertension. Portal hypertension due to an increase in sinusoidal pressure activates vasodilatory mechanisms. In advanced stages of cirrhosis, arteriolar vasodilation causes underfilling of systemic arterial vascular space. This event, through a decrease in effective blood volume, leads to a drop in arterial pressure. Consequently, baroreceptor-mediated activation of renin-angiotensin aldosterone system, sympathetic nervous system and nonosmotic release of antidiuretic hormone occur to restore the normal blood homeostasis. These events cause further retention of renal sodium and fluid. Splanchnic vasodilation increases splanchnic lymph production, exceeding the lymph transportation system capacity, and leads to lymph leakage into the peritoneal cavity. Persistent renal sodium and fluid retention, alongside increased splanchnic vascular permeability in addition to lymph leakage into the peritoneal cavity, play a major role in a sustained ascites formation.
Thiazolidinediones (TZD's), such as rosiglitazone, are peroxisome proliferator-activated receptor (PPAR) gamma agonist agents used for the treatment of type-2 diabetes and are widely prescribed. Unfortunately, fluid retention has emerged as the most common and serious side-effect of TZD's and has become the most frequent cause of discontinuation of therapy. The incidence of TZD-induced fluid retention ranges from 7% in monotherapy and to as high as 15% when combined with insulin (Yan, T., Soodvilai, S., PPAR Research volume 2008, article ID 943614). The mechanisms for such side-effects are not fully understood but may be related in Na and fluid re-absorption in the kidney. However TZD-induced fluid retention is resistant to loop diuretics or thiazide diuretics, and combination of peroxisome proliferator-activated receptor (PPAR) alpha with PPAR gamma agonists, which were proposed to reduce such fluid overload, are associated with major adverse cardiovascular events.
In view of the foregoing, it is recognized that salt and fluid accumulation contribute to the morbidity and mortality of many diseases, including heart failure (in particular, congestive heart failure), chronic kidney disease, end-stage renal disease, liver disease and the like. It is also accepted that salt and fluid accumulation are risk factors for hypertension. Accordingly, there is a clear need for a medicament that, when administered to a patient in need, would result in a reduction in sodium retention, fluid retention, or preferably both. Such a medicament would more preferably also not involve or otherwise impair renal mechanisms of fluid/Na homeostasis.
One option to consider for treating excessive fluid overload is to induce diarrhea. Diarrhea may be triggered by several agents including, for example, laxatives such as sorbitol, polyethyleneglycol, bisacodyl and phenolphthaleine. Sorbitol and polyethyleneglycol triggers osmotic diarrhea with low levels of secreted electrolytes; thus, their utility in removing sodium salt from the GI tract is limited. The mechanism of action of phenolphthalein is not clearly established, but is thought to be caused by inhibition of the Na/K ATPase and the Cl/HCO3 anion exchanger and stimulation of electrogenic anion secretion (see, e.g., Eherer, A. J., C. A. Santa Ana, J. Porter, and J. S. Fordtran, 1993, Gastroenterology, v. 104, no. 4, p. 1007-1012). However, some laxatives, such as phenolphthalein, are not viable options for the chronic treatment of fluid overload, due to the potential risk of carcinogenicity in humans. Furthermore, laxatives may not be used chronically, as they have been shown to be an irritant and cause mucosal damage. Accordingly, it should also be recognized that the induction of chronic diarrhea as part of an effort to control salt and fluid overload would be an undesired treatment modality for most patients. Any medicament utilizing the GI tract for this purpose would therefore need to control diarrhea in order to be of practical benefit.
One approach for the treatment of mild diarrhea is the administration of a fluid-absorbing polymer, such as the natural plant fiber psyllium. Polymeric materials, and more specifically hydrogel polymers, may also be used for the removal of fluid from the gastrointestinal (GI) tract. The use of such polymers is described in, for example, U.S. Pat. No. 4,470,975 and No. 6,908,609, the entire contents of which are incorporated herein by reference for all relevant and consistent purposes. However, for such polymers to effectively remove significant quantities of fluid, they must desirably resist the static and osmotic pressure range existing in the GI tract. Many mammals, including humans, make a soft feces with a water content of about 70%, and do so by transporting fluid against the high hydraulic resistance imposed by the fecal mass. Several studies show that the pressure required to dehydrate feces from about 80% to about 60% is between about 500 kPa and about 1000 kPa (i.e., about 5 to about 10 atm). (See, e.g., McKie, A. T., W. Powrie, and R. J. Naftalin, 1990, Am J Physiol, v. 258, no. 3 Pt 1, p. G391-G394; Bleakman, D., and R. J. Naftalin, 1990, Am J Physiol, v. 258, no. 3 Pt 1, p. G377-G390; Zammit, P. S., M. Mendizabal, and R. J. Naftalin, 1994, J Physiol, v. 477 (Pt 3), p. 539-548.) However, the static pressure measured intraluminally is usually between about 6 kPa and about 15 kPa. The rather high pressure needed to dehydrate feces is essentially due to an osmotic process and not a mechanical process produced by muscular forces. The osmotic pressure arises from the active transport of salt across the colonic mucosa that ultimately produces a hypertonic fluid absorption. The osmotic gradient produced drives fluid from the lumen to the serosal side of the mucosa. Fluid-absorbing polymers, such as those described in for example U.S. Pat. Nos. 4,470,975 and 6,908,609, may not be able to sustain such pressure. Such polymers may collapse in a normal colon where the salt absorption process is intact, hence removing a modest quantity of fluid and thereby salt.
Synthetic polymers that bind sodium have also been described. For example, ion-exchange polymeric resins, such as Dowex-type cation exchange resins, have been known since about the 1950's. However, with the exception of Kayexalate™ (or Kionex™), which is a polystyrene sulfonate salt approved for the treatment of hyperkalemia, cation exchange resins have very limited use as drugs, due at least in part to their limited capacity and poor cation binding selectivity. Additionally, during the ion-exchange process, the resins may release a stoichiometric amount of exogenous cations (e.g., H, K, Ca), which may in turn potentially cause acidosis (H), hyperkalemia (K) or contribute to vascular calcification (Ca). Such resins may also cause constipation.
Gastrointestinal Tract Disorders
Constipation is characterized by infrequent and difficult passage of stool and becomes chronic when a patient suffers specified symptoms for over 12 non-consecutive weeks within a 12-month period. Chronic constipation is idiopathic if it is not caused by other diseases or by use of medications. An evidence-based approach to the management of chronic constipation in North America (Brandt et al., 2005, Am. J. Gastroenterol. 100(Suppl.1):S5-S21) revealed that prevalence is approximately 15% of the general population. Constipation is reported more commonly in women, the elderly, non-whites, and individuals from lower socioeconomic groups.
Irritable bowel syndrome (IBS) is a common GI disorder associated with alterations in motility, secretion and visceral sensation. A range of clinical symptoms characterizes this disorder, including stool frequency and form, abdominal pain and bloating. The recognition of clinical symptoms of IBS are yet to be defined, but it is now common to refer to diarrhea-predominant IBS (D-IBS) and constipation-predominant IBS (C-IBS), wherein D-IBS is defined as continuous passage of loose or watery stools and C-IBS as a group of functional disorders which present as difficult, infrequent or seemingly incomplete defecation. The pathophysiology of IBS is not fully understood, and a number of mechanisms have been suggested. Visceral hypersensitivity is often considered to play a major etiologic role and has been proposed to be a biological marker even useful to discriminate IBS from other causes of abdominal pain. In a recent clinical study (Posserud, I. et al, Gastroenterology, 2007; 133:1113-1123) IBS patients were submitted to a visceral sensitivity test (Balloon distention) and compared with healthy subjects. It revealed that 61% of the IBS patients had an altered visceral perception as measured by pain and discomfort threshold. Other reviews have documented the role of visceral hypersensitivity in abdominal pain symptomatic of various gastrointestinal tract disorders (Akbar, A, et al, Aliment. Pharmaco. Ther., 2009, 30, 423-435; Bueno et al., Neurogastroenterol Motility (2007) 19 (suppl.1), 89-119). Colonic and rectal distention have been widely used as a tool to assess visceral sensitivity in animal and human studies. The type of stress used to induce visceral sensitivity varies upon the models (see for instance Eutamen, H Neurogastroenterol Motil. 2009 Aug. 25. [Epub ahead of print]), however stress such as Partial restraint stress (PRS) is a relatively mild, non-ulcerogenic model that is considered more representative of the IBS setting.
Constipation is commonly found in the geriatric population, particularly patients with osteoporosis who have to take calcium supplements. Calcium supplements have shown to be beneficial in ostoporotic patients to restore bone density but compliance is poor because of calcium-induced constipation effects.
Opioid-induced constipation (OIC) (also referred to as opioid-induced bowel dysfunction or opioid bowel dysfunction (OBD)) is a common adverse effect associated with opioid therapy. OIC is commonly described as constipation; however, it is a constellation of adverse gastrointestinal (GI) effects, which also includes abdominal cramping, bloating, and gastroesophageal reflux. Patients with cancer may have disease-related constipation, which is usually worsened by opioid therapy. However, OIC is not limited to cancer patients. A recent survey of patients taking opioid therapy for pain of non-cancer origin found that approximately 40% of patients experienced constipation related to opioid therapy (<3 complete bowel movements per week) compared with 7.6% in a control group. Of subjects who required laxative therapy, only 46% of opioid-treated patients (control subjects, 84%) reported achieving the desired treatment results >50% of the time (Pappagallo, 2001, Am. J. Surg. 182(5A Suppl.):11S-18S).
Some patients suffering from chronic idiopathic constipation can be successfully treated with lifestyle modification, dietary changes and increased fluid and fiber intake, and these treatments are generally tried first. For patients who fail to respond to these approaches, physicians typically recommend laxatives, most of which are available over-the-counter. Use of laxatives provided over-the-counter is judged inefficient by about half of the patients (Johanson and Kralstein, 2007, Aliment. Pharmacol. Ther. 25(5):599-608). Other therapeutic options currently prescribed or in clinical development for the treatment of IBS and chronic constipation including OIC are described in, for example: Chang et al., 2006, Curr. Teat. Options Gastroenterol. 9(4):314-323; Gershon and Tack, 2007, Gastroenterology 132(1):397-414; and, Hammerle and Surawicz, 2008, World J. Gastroenterol. 14(17):2639-2649. Such treatments include but are not limited to serotonin receptor ligands, chloride channel activators, opioid receptor antagonists, guanylate-cyclase receptor agonists and nucleotide P2Y(2) receptor agonists. Many of these treatment options are inadequate, as they may be habit forming, ineffective in some patients, may cause long term adverse effects, or otherwise are less than optimal.
Na+/H+ Exchanger (NHE) Inhibitors
A major function of the GI tract is to maintain water/Na homeostasis by absorbing virtually all water and Na to which the GI tract is exposed. The epithelial layer covering the apical surface of the mammalian colon is a typical electrolyte-transporting epithelium, which is able to move large quantities of salt and water in both directions across the mucosa. For example, each day the GI tract processes about 9 liters of fluid and about 800 meq of Na. (See, e.g., Zachos et al., Molecular physiology of intestinal Na+/H+ exchange; Annu. Rev. Physiol., v. 67, p. 411-443 (2005).) Only about 1.5 liters of this fluid and about 150 meq of this sodium originates from ingestion; rather, the majority of the fluid (e.g., about 7.5 liters) and sodium (about 650 meq) is secreted via the GI organs as part of digestion. The GI tract therefore represents a viable target for modulating systemic sodium and fluid levels.
Many reviews have been published on the physiology and secretory and/or absorption mechanisms of the GI tract (see, e.g., Kunzelmann et al., Electrolyte transport in the mammalian colon: mechanisms and implications for disease; Physiol. Rev., v. 82, no. 1, p. 245-289 (2002); Geibel, J. P.; Secretion and absorption by colonic crypts; Annu. Rev. Physiol, v. 67, p. 471-490 (2005); Zachos et al., supra; Kiela, P. R. et al., Apical NA+/H+ exchangers in the mammalian gastrointestinal tract; J. Physiol. Pharmacol., v. 57 Suppl. 7, p. 51-79 (2006)). The two main mechanisms of Na absorption are electroneutral and electrogenic transport. Electroneutral transport is essentially due to the Na+/H+ antiport NHE (e.g., NHE-3) and is responsible for the bulk of Na absorption. Electrogenic transport is provided by the epithelium sodium channel (“ENaC”). Electroneutral transport is located primarily in the ileal segment and proximal colon and electrogenic transport is located in the distal colon.
Plasma membrane NHEs contribute to maintenance of intracellular pH and volume, transcellular absorption of NaCl and NaHCO3, and fluid balance carried out by epithelial cells, especially in the kidney, intestine, gallbladder, and salivary glands, as well as regulation of systemic pH. There exists a body of literature devoted to the role and clinical intervention on systemic NHEs to treat disorders related to ischemia and reperfusion for cardioprotection or renal protection. Nine isoforms of NHEs have been identified (Kiela, P. R., et al.; Apical NA+/H+ exchangers in the mammalian gastrointestinal tract; J. Physiol. Pharmacol., v. 57 Suppl 7, p. 51-79 (2006)), of which NHE-2, NHE-3 and NHE-8 are expressed on the apical side of the GI tract, with NHE-3 providing a larger contribution to transport. Another, yet to be identified, Cl-dependant NHE has been identified in the crypt of rat cells. In addition, much research has been devoted to identifying inhibitors of NHEs. The primary targets of such research have been NHE-1 and NHE-3. Small molecule NHE inhibitors are, for example, described in: U.S. Pat. Nos. 5,866,610; 6,399,824; 6,911,453; 6,703,405; 6,005,010; 6,736,705; 6,887,870; 6,737,423; 7,326,705; 5,824,691 (WO 94/026709); U.S. Pat. No. 6,399,824 (WO 02/024637); U.S. Pat. Pub. Nos. 2004/0039001 (WO 02/020496); 2005/0020612 (WO 03/055490); 2004/0113396 (WO 03/051866); 2005/0020612; 2005/0054705; 2008/0194621; 2007/0225323; 2004/0039001; 2004/0224965; 2005/0113396; 2007/0135383; 2007/0135385; 2005/0244367; 2007/0270414; International Publication Nos. WO 01/072742; WO 01021582 (CA2387529); WO 97/024113 (CA02241531) and European Pat. No. EP0744397 (CA2177007); all of which are incorporated herein by reference in their entirety for all relevant and consistent purposes. However, to-date, such research has failed to develop or recognize the value or importance of NHE inhibitors that are not absorbed (i.e., not systemic) and target the gastrointestinal tract. Such inhibitors could be utilized in the treatment of disorders associated with fluid retention and salt overload and in the treatment of GI tract disorders, including the treatment or reduction of pain associated with a gastrointestinal tract disorder. Such inhibitors would be particular advantageous because they could be delivered with reduced fear of systemic on-target or off-target effects (e.g., little or no risk of renal involvement or other systemic effects.
Accordingly, while progress has been made in the foregoing fields, there remains a need in the art for novel compounds for use in the disorders associated with fluid retention and salt overload and in the treatment of gastrointestinal tract disorders, including the treatment or reduction of pain associated with a gastrointestinal tract disorder. The present invention fulfills this need and provides further related advantages.